This invention relates to fabrication of a bipolar integrated circuit, and more specifically to fabrication of a bipolar integrated circuit on a gallium arsenide substrate.
GaAs MESFET devices have been quite successful in applications such as microwave and high speed digital circuits. This success of GaAs MESFETs has been due mainly to high electron mobility of GaAs and existence of semi-insulating GaAs substrates. Even in view of these successes, and a long period of development, the application of GaAs MESFETs to anything more than MSI logic in digital circuitry has not come about. Some of the reasons for this are: difficult circuit modeling and design, poor threshold voltage control, and sensitivity of MESFET operation to load conditions.
Silicon bipolar technology is well known for its advantages in terms of uniform threshold, noise immunity, and circuit design ease, which make it well suited for custom LSI such as gate arrays. However, the GaAs bipolar transistor has received little attention, due in great part to poor results obtained in the early period of research on GaAs devices. These poor results were generally caused by problems with diffusion techniques that were not accurate enough to control required device characteristics. The main problem encountered with the diffusion techniques is that dopant profiles could not be adequately established in a material using diffusion. Dopant concentrations tend to fall off with depth, and the concentration at a given depth is not accurately predictable.
The difficulty with the diffusion techniques was caused by inadequate control of the doping profile of the base region. In the prior art, a zinc or magnesium diffusion would be used to form a P-type base region, but adequate control of the doping profile was not practicable. Implantation avoids this problem, since the doping profile can be controlled to be more even within the base region, as will be further discussed below. For background, see Glaser and Subak-Sharpe, Integrated Circuit Engineering 1977, which is hereby incorporated by reference.
However, recent success of ion-implantation compared to earlier diffusion techniques means that critical junction depths and doping concentrations, which significantly affect bipolar IC design and performance, can now be precisely controlled. Some problems will remain with the GaAs bipolar device, such as low hole mobility and low maximum donor concentration, but novel design of devices and circuitry should make possible the full utilization of GaAs bipolar technology potential for high-speed digital applications.